This invention relates to improvements in binary coded signal correlation in a relative phase-shift modulated carrier communication system, and more particularly to the use of a variable length shift register for adjustment to compensate for Doppler effects on the incoming carrier and to a technique for implementing the variable shift register.
Phase modulation of binary data on a carrier wave (electromagnetic or acoustic) is commonly used since the phase-shift of the carrier wave from one bit period to another is relatively easy to demodulate. In some systems the carrier is actually a subcarrier modulated on a steady-phase carrier. The latter is tracked by the receiver and used to generate a reference signal of the frequency of the subcarrier for demodulation purposes (i.e. for phase detection). However, that and other techniques for extracting a phase reference from the signal received is exploited only in electromagnetic wave transmissions due to limitations of transducers employed in acoustic wave transmissions.
In systems which do not somehow transmit a phase reference, relative binary phase modulation is employed to encode the binary data on the carrier. Each binary digit may consist of a precise number of cycles, such as seven, with a phase reversal from one digit to the next to indicate a binary 1 or 0. Assuming that phase reversal is selected to represent a binary 0, transmission of either phase (0.degree. or 180.degree.) may be selected for the first binary digit (bit). If a 0 is the first bit, the phase of the carrier is reversed (inverted) for the next bit period; otherwise it is not reversed so that successive bit periods of the same phase (0.degree. or 180.degree.) represent a data bit 1. The opposite convention could as well be selected to represent a data bit 1 by reversing the phase.
To demodulate such a relative phase-shift modulated signal, the carrier received during each bit period is stored and compared with the phase of the carrier during the next period. Each data bit period thus provides a reference phase for the next data bit period. A problem with relative binary phase-shift modulation is demodulation in the presence of Doppler shift of the carrier due to relative motion between the transmitter and receiver, particularly when the Doppler shift is not known in advance, and cannot be independently determined.
To understand this problem which arises because of Doppler shift, assume a binary bit period of the modulated carrier to consist of seven cycles of the carrier. With a positive Doppler shift, the period of the seven cycles decreases while for a negative Doppler shift the period increases. Consequently, if an attempt is made to demodulate by comparing the phase of one bit period with the phase of a succeeding bit period, an error will occur because one bit period being compared with the other will be overlapping with another bit period, and this error would be cumulative such that after demodulating a 10 or 20 bit word, the demodulaton of the last few bits would be totally unreliable. The error is, of course, the offset in the comparison of cycles in one data bit period with cycles of a succeeding data bit period. But even assuming that somehow demodulation has been properly effected in the presence of significant Doppler phase shift, there is still a problem in proper identification of the bits in the resulting (demodulated) signal.
To appreciate this last problem of identifying a properly demodulated signal in the presence of Doppler shift, consider trying to decode the first N bits of a coded transmission by comparison with an N bit code word stored at the receiver. It is common practice to transmit such a code word for comparison in order to determine when the first bit of a following message occurs, or to simply discriminate against noise where only the code word is transmitted, as when a code word is used to cause a receiver to respond in some way. An example might be to shut off a valve in a blow-out prevention system for offshore drilling platforms. Serial or parallel comparison of the coded signal received with a stored replica would be impossible, unless the replica is somehow compressed or expanded by an amount approximately equal to the amount the data bit periods of the coded signal have been compressed or expanded due to any Doppler shift caused by relative motion between the transmitter and the receiver.
In a copending application U.S. Ser. No. 604,085 filed concurrently by Michael G. Winters, and assigned to the Assignee of the present application, there is disclosed a technique for demodulating binary phase-shift modulated carrier signals and for detecting a binary code of N digits in the demodulated binary signals with greatly improved correlation through compensation for any phase shift in the carrier signal. Briefly, the technique consists of first subjecting a relative binary phase-shift modulated carrier to zero cross-over detection to produce a square-wave signal of the same frequency and phase as the carrier input signal. The resulting square-wave signal is then sampled at a predetermined rate to produce a number, S, of sample pulses during each nominal data bit period, each sample pulse being of one voltage level representing a binary 1 for one phase, and of another voltage level representing a binary 0 for the reversed phase of the sampled signal.
The resulting train of sample bits are applied to a comparator for phase comparison of each sample bit with a sample bit delayed by the nominal data bit period. Each sample bit is similarly applied to additional separate comparators for phase comparison with sample bits delayed by amounts which are greater (+.DELTA.S) than and less (-.DELTA.S) than the nominal data bit period (S) to compensate for any Doppler effect which makes the received data bit periods expand or contract. These amounts, .+-..DELTA.S, are predetermined to be approximately correct for the average positive and negative Doppler shifts expected. The train of sample bits per data bit delayed by amounts greater than and less than the nominal data bit period delay will thus be S.+-..DELTA.S, where .DELTA.S is the number of samples which make up the period by which the data bit period has expanded or contracted.
The comparison of sample bits with delayed sample bits produces a bit 1 at a comparator output each time the two bits compared are identical; otherwise a bit 0 is produced. The result is effective demodulation free of error out of the comparator associated with the channel which provides a delay of S sample bit periods when there has been no Doppler shift on the carrier signal received. If there has been a Doppler shift, there will be an error in the demodulated output of this nominal delay channel, but there will be less error out of one of the other two delay channels, which one depending upon whether the Doppler shift has been positive or negative. If there has been a positive Doppler shift, the comparison bits from the shorter delay channel will be predominantly all ones or zeros during a contracted data bit period, and if there has been a negative Doppler shift, the comparison bits from the longer delay channel will be predominantly all ones or zeros during the expanded data bit period.
The outputs of the three comparators are separately processed through separate delay lines of a length effectively equal to the number, N, of data bits in a demodulated code word times the length of the associated comparison delay channel in terms of sample bits in the nominal, expanded and contracted data bit period, i.e. of a length effectively equal to N.sup.. S, N(S+.DELTA.S) and N(S-.DELTA.S). Each delay line permits continuous comparison of the stream of sample bits with the static replica of the demodulated code word, each binary digit of the replica being compared in parallel in a separate comparator with a number of successive sample bits equal to the number S, S+.DELTA.S, and S-.DELTA.S. When a demodulated bit agrees with a replica bit, the result of the comparison is a detection bit 1; otherwise it is a detection bit 0. Detection bits from the three comparators are separately summed over the last S, S+.DELTA.S, and S-.DELTA.S samples, such that after summing for a sufficient time, the sum is always of N.sup.. S, N(S+.DELTA.S), and N(S-.DELTA.S) possible detection bits.
The sum from each separate summing means will reach a peak when the demodulated input signal of all N data bits represented by the S, S+.DELTA.S, or S-.DELTA.S sample bits are properly registered in the separate delay lines. If there has been no Doppler shift on the carrier signal, the nominal delay line having a length equal to N.sup.. S will cause its detection comparator to produce a peak greater than that from either of the other detection comparators. If there has been a positive or negative Doppler shift, one or the other of the remaining detection comparators will produce the largest peak. Consequently, the comparator producing the largest peak will be from a comparison of the received data bits in the delay line which more nearly matches the Doppler shift of the received signal. The channel of the largest sum may then be selected as the one having the highest probability of containing a correct Doppler shift compensation. The largest sum is tested to determine if it is greater than a predetermined value. If so it is determined that the coded word on the carrier has been demodulated and detected.